I. Field of the Invention
The present invention relates in general to varistors, and more particularly to such varistors having voltage rating characteristics which may be varied without varying the thickness of the varistor body.
II. Description of Related Art
Varistors, and especially metal oxide varistors, have gained widespread acceptance as devices for providing a nonlinear resistance function. The electrical characteristics of such voltage-dependent resistors are expressed in part by the relation: EQU I=(V/C).sup.n
where V is the voltage across the varistor, I is the current flowing through the varistor, C is a constant corresponding to the voltage at a given current, and the exponent n has a numerical value greater than 1. The value of n is calculated according to the following relation: ##EQU1## where V.sub.1 and V.sub.2 are the voltages at currents I.sub.1 and I.sub.2, respectively. The desired value for C depends upon the type of application in which the varistor is to be used. It is ordinarily desirable that the value of n be as large as possible, since this exponent determines the degree to which the varistor departs from Ohmic characteristics.
Although substantial effort on the part of many investigators has led to increasing understanding of the characteristics and methods of operation of metal oxide varistors, the device is nevertheless not completely understood. For this reason, many significant improvements in varistor operation are made more or less heuristically, and the reasons for the improvement or mechanism or the accomplishment thereof are not always known with complete certainty.
It is known, however, that the electrical properties of a varistor are determined primarily by the physical dimensions of the varistor body. The energy rating of a varistor is determined by the volume of the varistor body, the voltage rating of a varistor is determined by the thickness or current path length through the varistor body, and the current capability of the varistor is determined by the area of the varistor body measured normal to the direction of current flow.
The varistor body itself is composed essentially of a polycrystalline material. Each crystal grain boundary within the polycrystalline varistor body acts in a manner essentially similar to that of a diode. The voltage rating of such a varistor may therefore be controlled by changing the number of grain boundaries between the electrodes positioned on the surface of the varistor body. The voltage rating of such a varistor is typically increased merely by making the varistor body thicker, since this will increase the number of crystals, and consequently the number of crystal grain boundaries, between the electrodes on the varistor body surface.
This method of increasing the voltage rating of the varistor does, however, have several disadvantages. Increasing the thickness of the varistor body solely to increase the voltage rating of the varistor device is not an efficient utilization of varistor body material. In addition, increasing the thickness of the varistor body tends to increase the series resistance between electrodes, which is an undesirable characteristic in certain applications. Moreover, in those applications in which the varistor is of the surface-mounted variety (i.e., the varistor input and output terminals are mounted on the same surface of the device), it is generally desirable that the varistor device lie flat. That is, surface-mounted varistors are preferably designed to be deposited in position on a printed circuit board or the like so that, once deposited, they will remain in position and will not roll over. If the vertical thickness of the varistor device is too large, the varistor will not be able to maintain a stable position on the circuit board and will instead tend to fall over.
Varistors in which both the input and output terminals are positioned on the same major surface of the varistor body are generally referred to as "surface mount" varistors. The voltage rating of such surface mount varistors is primarily a surface property of the varistor device (i.e., it is dependent primarily upon the current flowing along the surface of the device), rather than a bulk property (i.e., dependent primarily upon the current flowing through the thickness of the varistor body). The voltage rating of such surface mount varistors may be increased without increasing the thickness of the varistor body. This may typically be accomplished merely by increasing the distance separating the electrodes on the same major surface of the varistor.
However, there are certain disadvantages associated with surface mount varistors which limits the use of such varistors in certain applications. One such disadvantage is the fact that surface mount varistors, in general, cannot absorb relatively large amounts of energy. Surface mount varistors also tend to be prone to a number of unwanted surface effects. For example, water, humidity, or fingerprints applied to the surface of the varistor are known to have adverse effects upon the operation of the device. These adverse effects are, in general, not experienced by bulk varistor devices.
It is an object, therefore, of the present invention to provide a varistor having a voltage rating which may be varied without varying the thickness of the varistor body.
It is a further object of the present invention to provide a surface mount varistor having a voltage rating which is not primarily a surface property of the device.
It is another object of the present invention to provide a varistor which more efficiently utilizes varistor body material.